α-Olefin oligomers having a double bond and 4 to 24 carbon atoms are useful substances widely used as monomer materials for olefinic polymers, comonomers for various high polymers and materials for plasticizers and surfactants. The α-olefin oligomers are produced, in general, by oligomerization of α-olefins used as the raw materials in the presence of a Ziegler-based catalyst. Since the oligomerization of an α-olefin is an exothermic reaction, it is necessary that the oligomerization be conducted while the generated heat is suitably removed when a continuous operation is conducted in the industrial scale. Examples of the process for removing the generated heat include the process comprising cooling by using a jacket of the reactor and the process comprising forming a circulating flow route from the reactor to an external heat exchanger and then back to the reactor and removing the generated heat in the external heat exchanger (hereinafter, this process will be occasionally referred to as the externally circulating heat exchanger process).
On the other hand, it is inevitable in the oligomerization that a considerable amount of polymers are formed as byproducts. The polymers formed as byproducts occasionally cause problems in the production in that the polymers adhere to the reactor and other apparatuses for the production and the apparatuses are clogged.
Among the above processes for removing the generated heat, the externally circulating heat exchanger process is more frequently used since a greater heat transfer area can be obtained. When this process is used, the above problem of precipitation of polymers inside the heat exchanger and clogging of the circulating line of the apparatuses frequently arises.